move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/telegram/downloader/parallel.go

@@ -0,0 +1,83 @@
+package downloader
+
+import (
+	"context"
+	"io"
+	"sync"
+
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/syncio"
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tdsync"
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+// nolint:gocognit
+func (d *Downloader) parallel(
+	ctx context.Context, r *reader,
+	threads int, w io.WriterAt,
+) (tg.StorageFileTypeClass, error) {
+	var typ tg.StorageFileTypeClass
+	typOnce := &sync.Once{}
+
+	ready := tdsync.NewReady()
+	g := tdsync.NewCancellableGroup(ctx)
+	toWrite := make(chan block, threads)
+
+	stop := func(t tg.StorageFileTypeClass) {
+		typOnce.Do(func() {
+			typ = t
+		})
+		ready.Signal()
+	}
+
+	// Download loop
+	g.Go(func(ctx context.Context) error {
+		downloads := tdsync.NewCancellableGroup(ctx)
+		defer close(toWrite)
+
+		for i := 0; i < threads; i++ {
+			downloads.Go(func(ctx context.Context) error {
+				for {
+					select {
+					case <-ctx.Done():
+						return ctx.Err()
+					case <-ready.Ready():
+						return nil
+					default:
+					}
+
+					b, err := r.Next(ctx)
+					if err != nil {
+						return errors.Wrap(err, "get file")
+					}
+
+					// If returned chunk is zero, that means we read all file.
+					n := len(b.data)
+					if n < 1 {
+						stop(b.tag)
+						return nil
+					}
+
+					select {
+					case <-ctx.Done():
+						return ctx.Err()
+					case toWrite <- b:
+					}
+
+					if b.last() {
+						stop(b.tag)
+						return nil
+					}
+				}
+			})
+		}
+
+		return downloads.Wait()
+	})
+
+	// Write loop
+	g.Go(writeAtLoop(syncio.NewWriterAt(w), toWrite))
+
+	return typ, g.Wait()
+}